<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>画板</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        #canvas {
            display: block;
        }
    </style>
</head>

<body>
<canvas id="canvas" width="100" height="100"></canvas>
<script>
  let canvas = document.getElementById("canvas");
  canvas.width = document.documentElement.clientWidth;
  canvas.height = document.documentElement.clientHeight;
  let ctx = canvas.getContext("2d");
  ctx.fillStyle = "black";
  ctx.strokeStyle = 'black';
  ctx.lineWidth = 8;
  // 绘制每一条线段末端的属性
  ctx.lineCap = "round";

  let painting = false;
  let last;
  var isTouchDevice = 'ontouchstart' in document.documentElement;
  if (isTouchDevice) {
    // 手机触屏模式下，手指点下
    canvas.ontouchstart = (e) => {
      let x = e.touches[0].clientX;
      let y = e.touches[0].clientY;
      last = [x, y];
    };

    // 手机触屏模式下，手指按住屏幕移动
    canvas.ontouchmove = (e) => {
      let x = e.touches[0].clientX;
      let y = e.touches[0].clientY;
      drawLine(last[0], last[1], x, y);
      last = [x, y];
    };
  } else {
    // PC端鼠标按下
    canvas.onmousedown = (e) => {
      painting = true;
      last = [e.clientX, e.clientY];
    }

    // PC端鼠标按住移动
    canvas.onmousemove = (e) => {
      if (painting === true) {
        drawLine(last[0], last[1], e.clientX, e.clientY);
        last = [e.clientX, e.clientY];
      }
    }

    // PC端鼠标松开
    canvas.onmouseup = () => {
      painting = false;
    }
  }

  // 封装画线方法
  function drawLine(x1, y1, x2, y2) {
    ctx.beginPath();
    ctx.moveTo(x1, y1);
    ctx.lineTo(x2, y2);
    ctx.stroke();
  }

  document.body.addEventListener('touchmove', e => {
    if (!e._isScroller) {
      // 阻止默认事件
      e.preventDefault();
    }
  }, {passive: false});
</script>
</body>
</html>
